1. Field of the Invention
This invention relates to an objective lens of large relative aperture which is well corrected for high grade imaging performance over an extended focusing range, toward extremely near object distances.
2. Description of the Prior Art
General-purpose photographic objectives are aberration-corrected for the best imagery, usually to an infinitely distant object. On the other hand, macro objectives which find their principal use in closeup photography are designed in view of the variation of aberrations with focusing, to provide optimum imagery usually at a reproduction ratio 1:10 as a basis. However, it often occurs with objectives having a large relative aperture and/or a wide angular field that as the reproduction ratio increases, the aberration varies to a larger extent. Particularly, spherical aberration and asymmetric aberrations of the off-axis rays are remarkably deteriorated. This is because (1) the tendency of the on-axis pencil to diverge is stronger for a close object point than for an infinitely distant object point, so that the on-axis pencil tends to diverge near the diaphragm, and the resultant spherical aberration becomes too large to be compensated for by the design of the positive lens component that follows the diaphragm, and (2) because the principal ray of the off-axis pencil makes a smaller angle with the optical axis so that in the closeup position outward coma is often caused to be produced. Such provision of a large range of variation of aberrations with different production ratios, implies that the achievement of the optimization of aberration correction at the reproduction ratio 1:10 as a basis, does not assure the preservation of sufficiently high grade imagery for either an extremely close object, or an infinitely distant object. To prevent such loss in imaging performance due to the change of the reproduction ratio from the design value, it has been the common practice in the art to cause a part of the lens system or a lens component to float during focusing. As the objective of the type described above mention may be made of those already known in U.S. Pat. Nos. 2,537,912, 3,884,557, and 4,260,223, for example.
Generally speaking, the conventional floating method gives rise to the problems that the focusing mechanism is made complicated, that the mechanical strain limits the available amount of floating movement to an insufficient level, and that the floating movement causes the interval between the principal points of the lens components to be widened, thus largely varying the focal length of the entire system.